1. Field of the Invention
This invention relates generally to an inspection device that generates an electromagnetic field for inspecting a bore and, more particularly, to a non-destructive testing system including an inspection device having orthogonal windings that provide multidirectional electromagnetic fields for inspecting bores in large valves for defects.
2. Discussion of the Related Art
Steam turbines are large machines that include many interconnected parts for converting thermal energy from steam to rotational energy to perform work. Some of these parts are large valves having bores that control fluid flow at various locations in the turbine in a manner that is understood by those skilled in the art. Because the valves operate in a very harsh thermal environment, the valve bores typically need to be periodically inspected for wear, defects and other discontinuities, such as surface induced cracks, that could detrimentally affect the operation of the turbine. Therefore, it is known in the art to periodically remove the valves and other components from the turbine and perform various maintenance procedures, such as during machine refurbishing, in a laboratory environment to inspect the valve bores for such defects.
Non-destructive testing of valve bores using a magnetic particle inspection process and electromagnetic fields during maintenance procedures is known in the art. In one know inspection process, an elongated cable or rod is inserted into the bore of the valve, where the rod includes a coil that is able to carry a current flow. The rod is generally inserted down the center of the bore to evenly provide the electromagnetic field over the entire circumference of the bore. The current flow in the coil generates an electromagnetic field around the coil that interacts with the ferroelectric valve structure through which the bore passes. The electromagnetic field causes currents to be induced in the valve structure proximate the bore, and if a discontinuity exists in the bore, the current and associated magnetic field cause a magnetic hysteresis loss at the discontinuity, which attracts iron or other magnetic particles. The valve body would typically be electrically coupled to a grounding line to allow the current flow in the valve structure. A solution including a suitable dye and suspended magnetic particles is provided on the internal surface of the bore. If a discontinuity exists, the magnetic particles collect at the discontinuity as a result of the hysteresis loss, which becomes more visible as a result of the higher intensity of the dye at those locations. Cameras, optical detectors, mirrors, etc. can be strategically placed relative to the bore so that this visual indication of a discontinuity can be observed by the technician performing the inspection.
The above described non-destructive test has limitations because of its ability to provide a suitable electromagnetic field strength in the valve structure that is necessary for generating the desired magnetic hysteresis loss at the discontinuities. Particularly, because the bores in many of these valves are quite large, the distance between the inspection device and the bore wall can be significant, where the field strength generated by the coil in the device drops off considerably before interacting with the valve structure. Making the inspection device larger in diameter has various drawbacks, including the need for having multiple inspection devices of different sizes, the increased size and weight of the inspection device, etc. Therefore, for many larger bores, the ability to detect certain defects or discontinuities is limited and sometimes not possible.
Further, the inspection device for the known testing system of this type includes a single coil that provides an electromagnetic field in a single direction relative to the bore. Particularly, the winding direction of the coil provides an electromagnetic field that induces a current flow in the valve structure in a direction along the length of the bore. For those defects that are perpendicular to this direction, the current would cause the magnetic particles to readily collect at the defect. However, for those defects that are parallel to the direction of the bore, where the direction of the current flow would be substantially parallel to the defect, the current flow would be limited in its ability to cause magnetic particles to collect at the defect.